This invention relates to systems for controlling the weights of individual charges of product or material including, but not limited to, powered or granular materials, liquids, slurrys, general solid material in divided form, and like food products, such as french fries, potato chips, shoestring potatoes, etc., for packaging into individual packages; however, the invention, in its broadest form, is suited for usage in other applications or environments, or in connection with the packaging of other types of materials than those specifically mentioned herein.
Weight control systems of this type basically adopt one of two approaches to control charge weight: one approach, often termed the "adjustable feed rate" approach, adjusts the feed rate at which material is accumulated; the other so-called "bulk-dribble" approach utilizes two feeders which operate at substantially constant feed rates, one feeder (termed the "bulk" feeder) operating at a high flow rate for rapidly bringing the charge almost to desired charge or "set" weight, the second feeder (termed the "dribble" feeder) operating at a low flow rate for accurately bringing the charge up to a set weight. In most practical applications, approximately 20-30 charges are formed and packaged per minute, charge rate depending upon charge material and the packaging and feeding apparatus employed. At these rates, however, present control systems, whether of the "adjustable feed rate" or "bulk-dribble" type, as the case may be, tend to drift, or lose accuracy for various reasons so that all charge weights obtained do not in fact correspond to a set weight but tend to fluctuate above and below set weight. Consequently, in order to maintain a given precentage of charges at weights in excess of a minimum acceptable weight (i.e., to prevent excessive numbers of underweight charges) set weight must exceed minimum acceptable weight by an amount sufficient to ensure that only a certain number of underweight charges will be produced, depending upon the shape of the distribution curve. Inasmuch as the typical distribution curve of percentage charges versus actual charge weights obtained with these systems is of low amplitude and wide band width with respect to set weight, however, they must be operated at set weights well above minimum acceptable weight. This technique results in substantial numbers of overweight charges and, hence, uneconomical product give-away.
It is desirable, therefore, to provide a weight control system which exhibits a more economical distribution curve and which, therefore, can be operated at set weights at or just above minimum acceptable weight with little or no product give-away. The "bulk-dribble" type of prior weight control system is more likely to exhibit such a distribution curve than the "adjustable feed rate" system because it maintains substantially constant feed rates in the bulk and dribble feeders and, for this reason, permits effective determination and control of undesirable effects which might go undetected, or be amplified by deliberate feed rate adjustment. Inasmuch as the bulk and dribble feeders operate at substantially constant feed rates, however, prior "bulk-dribble" systems yield charge accumulation times which may vary depending upon material flow conditions and other factors, with resultant uneconomical decline in accuracy of the charge weights obtained or idling of the packaging machine. Prior weight control systems of both the aforementioned types are uneconomical for still other reasons. Most are basically electromechanical in nature and typically employ servos for controlling charge weight in combination with external calibration weights for establishing set weight. In practical applications involving the charge accumulation rates mentioned previously, however, the servos cannot respond quickly enough to weight fluctuation, or tend to coast past set weight, while set weight, once established, cannot be adjusted readily for different charge weights, nor can it be calibrated with respect to actual system conditions. Prior weight control systems further are adversely affected by vibrational forces transmitted from vibrating material feeders, packaging machines etc., or by impact forces produced by material as it is dumped onto a scale or weigh cell, or by variations in material flow conditions.